Thread tensioning device

ABSTRACT

A system for tensioning thread in which an electromagnetic field is caused to pass through a pair of surfaces causing them to be attracted to each other. In the first embodiment, a ferromagnetic bobbin in a ferromagnetic bobbin case is caused to frictionally engage the bobbin case due to the attraction caused by the electromagnetic field, thereby tensioning thread removed from the bobbin. Additional tension is caused by the magnetic drag between the bobbin and case. A second embodiment involves passing a thread between a pair of plates which are attracted to each other by an electromagnetic field, thereby tensioning the thread. Tension is adjusted by varying the electromagnetic field.

Thomas [22] Filed:

[ 1 THREAD TENSIONING DEVICE [75] Inventor: Charles C, Thomas, Starkville, Miss.

[73] Assignee: Mississippi State University Development Foundation, Inc., State College, Miss.

Feb. 12, 1973 21 Appl. No.2 332,034

[52] US. Cl. 242/156, 112/254, 242/150 M,

- 310/93 [51] Int; Cl B65h 59/04, 1365b 59/22 [58] Field Of Search 242/156, 156.2, 150 M, 242/155 M, 45, 75.4, 129.8; 310/93; 1 112/255, 254

[56] References Cited UNITED STATES PATENTS 1,996,450 4/1935 Bes 242/75.4 X 2,543,475 2/1951 Slodysko 310/93 X 2,768,796 10/1956 Levine 242/155 M 2,907,535 10/1959 Mindheim et al.... 242/150 M 2,924,397 2/1960 Heppner 242/155 M 2,978,203 4/1961 Westhall et ah 242/155 M 3,100,091 8/1963 Mindheim et a1; 242/150 M [45'] Dec. 24, 1974 3,703,263 7 11/1972 Hunter 242/156X 3,724,409 4/1973 O1ney, Jr 242/155 M X FOREIGN PATENTS OR APPLICATIONS 160,469 3/1921 Great Britain 242/156 Primary Examiner-Stanley N. Gilreath Attorney, Agent, or FirmMillen, Raptes & White [5 7 ABSTRACT A system for tensioning thread in which an electromagnetic field is caused to pass through a pair of surfaces causing them to be attracted to each other. In the first embodiment, a ferromagnetic bobbin in a ferromagneti'c bobbin case is caused to frictionally engage the bobbin case due to the attraction caused by the electromagnetic field, thereby tensioning thread removed from the bobbin. Additional tension is caused by the magnetic drag between the bobbin and case. A second embodiment involves passing a thread between a pair of plates which are attracted to each otherby an electromagnetic field, thereby tensioning the thread. Tension is adjusted by varying the electromagnetic field.

16 Claims, 5 Drawing Figures 36 UPPER TENSION CONTROL PAIEmwummn 3 '8 56,233

sum 1 or 2 FIGS POWER SUPPLY SBNIHJVH HEIHlO 0i I THREAD TENSIONING DEVICE BACKGROUND OF THE INVENTION I ing a magnetic field producedby an electromagnet being taken from the bobbin of a lockstitch sewing machine. The term thread, as used herein, includes monofilament and polyfilament aswell. as spun fiber threads.

In the conventional lockstitch sewing machine, tensioning of the bobbin thread is accomplished after it leaves the bobbin by a flat spring mounted on the outside of the bobbin case which presses against-the thread as it is drawn from the bobbin. Tension is varied manually by adjustment of a screw bearing against thespring, which increases or decreases the spring force.

Adjustment of the tension can only be made by trial and error, due to variations in the thread used and the fabric being sewn. This is cumbersome and time consuming, particularly because the bobbin must be removed in order for the tension to be adjusted. Further- -more, in aconventional machine, adjustment of the bobbin tension must be repeated at frequent. intervals due to changes in tension caused by the accumulation of lint under the spring. Chain stitch sewing machines,

which include a broad. range of tackers, sergers, over- I through the bobbin, thus causing "the bobbin to bear against its case and thus tension the thread. Additional tension is caused by the magnetic drag between the ferromagnetic bobbin and the ferromagnetic bobbin case. The magnetic drag is the product of the interaction of the magnetic field of the electromagnet and the currents induced in the bobbin and case by the electro magnets magnetic field. This allows the tension to be adjusted, without the necessity of removing the bobbin, by varying the current supplied to the electromagnet.

The term ferromagnetic as used herein means formed I of or containing a ferrous material which is attracted magnetically by a magnetic field. A suitable bobbin containing ferromagnetic material is disclosed in Ser. No. l06,026,-now U.S. Pat. No. 3,7l6,202,'whose disclosure is incorporated by reference, which is drawn to a paper bobbin impregnated with ferromagnetic material. Alternatively, an all metal bobbin made of a ferromagnetic metal can be used.

lock machines and buttonhole machines, do not use a bobbin but nevertheless have manual spring tension controls for the thread. Proper control of tension is necessary in these machines to insure proper stitch formation and to minimize puckering of the fabric. In most cases, tension control on the thread is achievedby a spring which presses together two surfaces between which the thread is drawn. With such an arrangement, it is difficult to maintain a constant thread tension due to irregularities in thethreadand the accumulation of particles of lint from the thread between the surfaces.

Attempts have been made in the past tosolve the problems presented by conventional mechanical thread tensioners by electromagnetic means. See U.S. Pat. Nos. 2,293,748; 2,543,475; 2,724,065; 3,347,435; 3,408,832; and 3,433,398. For example, the use of a ferromagnetic bobbin rotating in a housing which includes a permanently magnetized element has been suggested. The rotation of the bobbin in the magnetic field sets up a magnetic drag which serves to tension the thread. However, this arrangement suffers from several serious defects. It is often difficult to obtain sufficient tension with such a system. Furthermore, adjustment can be obtained only by varying the airgap in the The use of an electromagnet eliminates the problems of lint collecting under a spring. The use of an electromagnet also makes it possible to achieve very high tension values. If desired, a feedback control system using a tensiontransducer can be used to maintain constant tension in spite of variations in the thread or other characteristics in the system. This can be especially important if extremely critical tension control is needed and it becomes necessary to vary the friction on the bobbin as the thread unwinds, thus decreasing the angular leverage through which the friction acts.

A dual-plate tensioner, such as that used in a lockstitch sewing machine, is also provided inaccordance with this invention. A ferromagnetic central shaft extending from the machine in the same position as the conventional spring tensioner is provided. Mounted on the shaft are a pair of conventional plates between magnetic circuit, which has the undesirable effect of even further weakening the drag-producing magnetic field. Another problem encountered in this system is its inability to concentrate the magnetic flux so as to obtain maximum tension.

SUMMARY OF THE INVENTION which the thread passes. Slidably mounted on the shaft is a ferromagnetic casing containing an electromagnetic coil. The coil is secured to the central shaft. The casing serves as a path for the magnetic flux, which is passed through the pair of plates, one of which is secured to the central shaft and the other to the casing,

causing them to be drawn toward each other, thereby tensioning the thread disposed between them.

The use of an electromagnetic tensioning device yields still another advantage. If it is desired to do so, all the tensioning devices can be connected to a central control console, so they can. be varied in unison instead of individually. A central operator can, by inspecting the output of the machines, ascertain what adjustments in tension on the upper or lower thread or both are necessary and make them simultaneously for all machines by adjustment of a single control going to all amrneters, thereby increasing or decreasing the tension on the corresponding thread tensioning devices on all of the machines. This is especially valuable in that, depending upon the material being sewn, the relative tensions between the upper and lower thread as well as the overall tension on the threads is important. It thus becomes a relatively simple operation to switch a plurality of ma-v chines from one material to another, in that it is only necessary to make a single adjustment instead of adjusting each machine separately. This is in contrast to the present practice of varying each of the plurality of machines separately by trial and error.

- FIG. 1 is a partial perspective view of a sewing machine provided-with the bobbin tensioner of this invention. v

FIG. 2 is a plan view, partially in section, of the bobbin tensioner'illustrated in FIG. 1, illustrating the operation of the device.

FIG. 3 is a perspective view, partially in section, of a dual-plate tensioner built in accordance with the present invention.

FIG. 4 is a plan view, partially in section, of the ten- 1 sioner illustrated in FIG. 3, showing the operation of the device.

FIG. 5 is a schematic diagram of a control circuit in a chain stitch sewing machine for controlling both bobbin tension with a device such as thatillustrated in FIG. 1 and upper thread tension with a device such as that illustrated in FIG. 3.

DETAILED DESCRIPTION OF THE DRAWINGS In the preferred embodiment, as illustrated in FIG. 1, bobbin thread tension is achieved through the use of an external electromagnet l, which produces a field in the bobbin and case combination which exits axially and enters radially as shown by the arrows in FIG. 2. The field is created by coil 2 and may be varied by changing the current flowing through the coil. Maximum efficiency in the use of the magnetic flux is achieved by providing electromagnet l with a bifurcated arm 4 having extensions 3a and 3b.

When it is desired, to activate the bobbin tensioner, direct current is passed through coil 2. This sets up a constant electromagnetic field which, as illustrated by arrow 11 in FIG. 2, passes through coil 2, end plate 6, rod 7, bobbin case 8, bobbin 9 and bifurcated arm 4.

The electromagnetic field causes bobbin 9 to bear against bobbin case 8, thus producing friction which provides the desired thread tension. Thread tension is a function of the friction caused by the electromagnetic field. Thus, thread tension can be regulated by varying the current passing through coil 2, which varies the electromagnetic field.

In order to provide for easy replacement of a bobbin after the thread therein has been exhausted, means are provided for rapidly moving the electromagnet 1 out of position. Electromagnet 1 is supported by rod 7 which is slidably mounted in sleeve 12, which is supported by bracket 13 which, in turn, is rigidly mounted on rail 14. Rail 14 serves as an additional guide for electromagnet l, and is secured to the sewing machine table 16. When it is desired to replace a bobbin, air cylinder 17 is air actuated, causing its plunger arm 18, which is attached to end plate 6, to be drawn into cylinder 18, thereby moving the electromagnet out of position and providing for easy access to the bobbin.

It has been found that truly effective control of bobbin tension in a lockstitch machine requires similar tension control of the upper thread. The device illustrated in FIGS. 3 and 4 provides proper upper thread tension control.

The upper thread tensioning device comprises a central ferromagnetic sleeve 21, which is slidably mounted. A coil 22 is wound around sleeve 21 and secured to it. The assembly is enclosed in casing 23 and end plate 24. Casing 23 is rigidly secured to outer plate the like.

26 and is slidably mounted on sleeve 21. End 14 is a projection of sleeve 21. When current is passed through the windings of coil 22, an electromagnetic field is set up which passes through sleeve 21, adjustment bolt 27, end plate 24, casing 23, outer plate 26, inner plate 28 and spacer 29, as illustrated by the arrows 40 in FIG. 4. This causes the casing 23 and end plate assembly 24, which is slidably mounted on sleeve 21, to be attracted toward inner plate 28 and spacer 29, thus applying a force thereto and thereby tensioning thread 31, which is disposed between plates 26 and 28. It is noted that plates 26 and 28 and spacer 29 need not all have to be ferromagnetic, as long as either plate 28 or spacer 29 is ferromagnetic. The force of attraction and its resultant tension can be varied'by varying the current passing through coil 22. The strength of the magnetic attraction can also be varied by adjusting bolt 27, which is threadably mounted in plate24 and bears against non-ferromagnetic plug 19 thereby increasing or decreasing the gap between end plate 24 and the assembly including sleeve 21 and plate 21a. Plate 21a is made of a ferromagnetic material. Plates 21a and 21b are preferably teflon coated for minimum friction.

As mentioned above, best results are obtained using a machine in which both upper and lower threads are tensioned by electromagnetic means. Such a system is illustrated in FIG. 5. Tensioners 32 and 33 are provided with direct current by a dc. power supply 34 having two independently adjustable outputs 41 and 42. Ammeters 36 and 37 are inserted in the linesto perform the function of measuring the current supplied to the tensioners and thus the tension supplied to the respective threads. Lines 43, 43a and 43a and 44, 44a and 44n go to other machines, thus allowing simultaneous adjustment of a plurality of machines by. a single pair of controls.

In practice, it has been found that best results are obtained when both thread tensioners are producing a minimum level of tension. However, after running the machine with a high degree of tension, it is often impossible to reduce the current without having a considerable amount of residual magnetism remaining in the circuit and thus, residual tension. Therefore, in the event that tension must be reduced a considerable degree, it is first necessary to remove the magnetism from the tensioning devices. A most convenient way of doing this is to first disconnect the dc. power sources and then apply a strong 60 cycle ac. current to the two electromagnets. This strong current is then reduced gradually to zero, resulting in de-magnetization of the ferromagnetic parts in the tensioners, thus eliminating residual magnetism and its resulting tension.

While the tensioning device has been described in the environment of a lockstitch sewing machine, it is to be understood that it can advantageously be employed in any system or apparatus requiring the tensioning of a thread, e.g., textile weaving and knitting machines and From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this l. A thread tensioning system for a sewing machine comprising means for tensioning the bobbin thread including:

a. a ferromagnetic bobbin, onto which said thread is wound;

bfla ferromagneticbobbin case for housing said bobbin, said bobbin case being rigidly and removably mounted to a support member;

c. an electromagnetic to provide a source of magnetic flux;

d. radial flux guiding mean'srigidly connected .to an end of said electromagnet for guiding magnetic flux radially between said bobbin and said bobbin case and said electromagnet;

e. axial flux guiding means rigidly connected to the other end of said electromagnet for guiding magnetic flux axially between said electromagnet and said bobbin and said bobbin case, wherein said radial flux .guiding means, said axial flux guiding means, said bobbin, said bobbin case and said electromagnet form a closed magnetic circuit;

means for supplying electrical current to said electromagnet to create said magnetic flux therein proportionately to the magnitude of said electrical current, thereby causing tension on the thread to be created by the combination of the magnetic drag resisting rotation of said bobbin in said magnetic flux and the frictional engagement between said bobbin and said bobbin case caused by said bobbin being urged against an interior wall of said bobbin case by said magnetic flux; and

means for varying said electrical current to vary said magnetic flux and thereby vary said thread tension.

2. The thread tensioning system of claim 1, wherein said radial flux guiding means includes a ferromagnetic bifurcated arm positioned radially with respect to said bobbin case, said arm having two extensions forming a semicircular wall around and in close proximity to said bobbin case.

3. The thread tensioning system of claim '1, wherein said axial flux guiding means includes:

a. an end plate having a lower end rigidly attached to the other end of said electromagnet;

b. a ferromagnetic rod having an end rigidly attached to an upper end of said end plate;

c. a sleeve for slideably supporting said rod;

d. a bracket for rigidly supporting said sleeve; and

e. a rail upon which said bracket is rigidly mounted and said end plate is slideably mounted for moving said electromagnet axially away from said bobbin case to provide access to said bobbin.

4. The thread tensioning system of claim 3 which further includes an air cylinder rigidly attached to said rail, said air cylinder having a plunger connected to said end plate for automatically moving said electromagnet axially away from said bobbin case.

5. The thread tensioning system of claim 1, wherein said means for supplying electrical current to said electromagnet includes:

a. a variable output dc. power supply; and

b. an ammeter connected in series with said power supply and said electromagnet, said ammeter being used to indicate the thread tension being applied by said thread tensioning system at any given time.

6. The thread tensioning system of claim 5, wherein said means for supplying electrical current is connected to a plurality of electromagnets of similar thread tensioningsystems for equalizing the bobbin thread tensions amongst such systems. I

7. The thread tensioning system of claim 1, which 5 further includes electromagnetic means for tensioning an upper needle thread comprising:

an electromagnetic assembly having a. a central ferromagnetic sleeve;

b. an electromagnetic coil wound around a portion of and rigidly secured to said sleeve;

c. a ferromagnetic first bushing end plate radially oriented and rigidly secured to one end of said sleeve proximate an end of said coil;

d. a second bushing end plate radially oriented and rigidly secured to an interior portion of said sleeve proximate the other end of said coil;

a ferromagnetic casing enclosing said electromagnetic assembly, said casing being slideably mounted upon said sleeve and said first and second bushing plates, with an end portion of said sleeve projecting therefrom;

a thread guide means assembly mounted upon said projecting portion of said sleeve including:

a. an outer plate rigidly attached to an end of said casing, and slideably mounted on said projecting portion of said sleeve;

b. an inner plate adjacent said outer plate;

'0. a spacer adjacent said inner plate, at least one of said inner plate and said spacer being ferromagnetic;

means for supplying electrical current to said electromagnetic assembly to create a magnetic flux passing through a magnetic circuit including said electromagnetic assembly, said sleeve, said casing, and at least one of said spacer and said inner plate, causing said casing to urge said outer plate against said inner plate to tension said needle thread therebetween, the magnitude of said tensioning'being proportional to the magnitude of said electrical current; and

means to vary said electrical current to said electromagnetic assembly to thereby vary said needle thread tension.

8. The thread tensioning system of claim 7 wherein said means for tensioning the needle thread includes:

a. said sleeve having a central bore;

b. a plug mounted in the core of said sleeve near the end of said sleeve proximate said first bushing;

c. an adjusting bolt threadably mounted through the other end of said casing proximate said first bushing, said screw bearing against said plug for increasing or decreasing the gap between said casing and said first bushing to vary the magnetic coupling therebetween.

9. The thread tensioning system of claim 7 wherein said first and second bushing end plates are Teflon coated for minimizing friction between said plates and said case.

10. The thread tensioning system of claim 7 wherein said means for supplying electrical current to said electromagnet coil of said means for tensioning the needle thread, includes: v

a. a variable output dc. power supply; and

b. an ammeter connected in series with said power supply and said electromagnet coil, said ammeter being used to indicate the tension being applied by 7 8 said thread tensioning means upon said needle said inner plate to tension said thread therebethread at any given time.v tween, the magnitude of said tensioning being pro- 11. The thread tensioning system of claim 10 wherein portional to the magnitude of said electrical cursaid means for supplying electrical current is connected rent; and V i r to a plurality of electromagnet coils of thread tensionmeans to vary said electrical current to said electroing systems similar to said needle thread tensioning magnetic assembly, thereby vary said thread means to equalize the needle thread tensions amongst i Such y 13. The thread tensioning apparatus of claim 12 An apparatus for tenslomng a thread comprising: wherein said thread tensioning apparatus further includes:

a. said sleeve having a central core;

b. a plug mounted in the core of said sleeve near the end of said sleeve proximate said first bushing;

c. an adjusting bolt threadably mountedthrough the other end of said casing proximate said first bushing, said screw bearing against said plug for increasing or decreasing the gap between said casing and said first bushing to vary the magnetic coupling therebetween.

14. The thread tensioning apparatus of claim 12 wherein said first and second bushing end plates are Teflon coated for minimizing friction between said an electromagnetic assembly having a. a central ferromagnetic sleeve;

b. anelectromagnetic coil wound around a portion of and rigidly secured to said sleeve;

c. a ferromagnetic first'bushing' end plate radially 15 oriented and rigidly secured to one end of said sleeve proximate an end of said coil;

d. a second bushing end plate radially oriented and rigidly secured to an interior portion of said sleeve proximate the other end of said coil;

a ferromagnetic casing enclosing said electromagnetic assembly, said casing being slideably mounted upon said sleeve and said first and second bushing plates, with an end portion of said sleeve plates and sad case t projecting therefrom; 15. The thread tensioning apparatus of claim 12 a thread guide means assembly mounted upon Said wherein said means for supplying electrical current to projecting portion of said Sleeve including; said electromagnet coil further includes: at an outer plate rigidly attached to an end of said a Variable Output P PPP Y} casing, d 1i b| mounted on Said projecting b. an ammeter connected in series wlth said power portion f id l supply and said electromagnet coil, said ammeter b, an inner plate dj id Outer l being used to indicate the tension being applied by c. a spacer adjacent said inner plate, at least one of Said t d tensioning means upon said thread at said inner plate and said spacer being ferromagy glven timenetic; 16. The thread tensioning apparatus of claim 15 means for supplying electrical current to said electrowherein Said means or supplying electrical Current is magnetic assembly to create a magnetic flux passconnected to a plurality of electromagnet coils of ing through a magnetic circuit including said electhread tensioning apparatus similar to said thread tentromagnetic assembly, said sleeve, said casing and sioning apparatus to equalize the thread tensions at least one of said spacer and said inner plate, amongst such apparatus.

causing said casing to urge said outer plate against 

1. A thread tensioning system for a sewing machine comprising means for tensioning the bobbin thread including: a. a ferromagnetic bobbin, onto which said thread is wound; b. a ferromagnetic bobbin case for housing said bobbin, said bobbin case being rigidly and removably mounted to a support member; c. an electromagnetic to provide a source of magnetic flux; d. radial flux guiding means rigidly connected to an end of said electromagnet for guiding magnetic flux radially between said bobbin and said bobbin case and said electromagnet; e. axial flux guiding means rigidly connected to the other end of said electromagnet for guiding magnetic flux axially between said electromagnet and said bobbin and said bobbin case, wherein said radial flux guiding means, said axial flux guiding means, said bobbin, said bobbin case and said electromagnet form a closed magnetic circuit; means for supplying electrical current to said electromagnet to create said magnetic flux therein proportionately to the magnitude of said electrical current, thereby causing tension on the thread to be created by the combination of the magnetic drag resisting rotation of said bobbin in said magnetic flux and the frictional engagement between said bobbin and said bobbin case caused by said bobbin being urged against an interior wall of said bobbin case by said magnetic flux; and means for varying said electrical current to vary said magnetic flux and thereby vary said thread tension.
 2. The thread tensioning system of claim 1, wherein said radial flux guiding means includes a ferromagnetic bifurcated arm positioned radially with respect to said bobbin case, said arm having two extensions forming a semicircular wall around and in close proximity to said bobbin case.
 3. The thread tensioning system of claim 1, wherein said axial flux guiding means includes: a. an end plate having a lower end rigidly attached to the other end of said electromagnet; b. a ferromagnetic rod having an end rigidly attached to an upper end of said end plate; c. a sleeve for slideably supporting said rod; d. a bracket for rigidly supporting said sleeve; and e. a rail upon which said bracket is rigidly mounted and said end plate is slideably mounted for moving said electromagnet axially away from said bobbin case to provide access to said bobbin.
 4. The thread tensioning system of claim 3 which further includes an air cylinder rigidly attached to said rail, said air cylinder having a plunger connected to said end plate for automatically moving said electromagnet axially away from said bobbin case.
 5. The thread tensioning system of claim 1, wherein said means for supplying electrical current to said electromagnet includes: a. a variable output d.c. power supply; and b. an ammeter connected in series with said power supply and said electromagnet, said ammeter being used to indicate the thread tension being applied by said thread tensioning system at any given time.
 6. The thread tensioning system of claim 5, wherein said means for supplying electrical current is connected to a plurality of electromagnets of similar thread tensioning systems for equalizing the bobbin thread tensions amongst such systems.
 7. The thread tensioning system of claim 1, which further includes electromagnetic means for tensioning an upper needle thread comprising: an electromagnetic assembly having a. a central ferromagnetic sleeve; b. an electromagnetic coil wound around a portion of and rigidly secured to said sleeve; c. a ferromagnetic first bushing end plate radially oriented and rigidly secured to one end of said sleeve proximate an end of said coil; d. a second bushing end plate radially oriented and rigidly secured to an interior portion of said sleeve proximate the other end of said coil; a ferromagnetic casing enclosing said electromagnetic assembly, said casing being slideably mounted upon said sleeve and said first and second bushing plates, with an end portion of said sleeve projecting therefrom; a thread guide means assembly mounted upon said projecting portion of said sleeve including: a. an outer plate rigidly attached to an end of said casing, and slideably mounted on said projecting portion of said sleeve; b. an inner plate adjacent said outer plate; c. a spacer adjacent said inner plate, at least one of said inner plate and said spacer being ferromagnetic; means for supplying electrical current to said electromagnetic assembly to create a magnetic flux passing through a magnetic circuit including said electromagnetic assembly, said sleeve, said casing, and at least one of said spacer and said inner plate, causing said casing to urge said outer plate against said inner plate to tension said needle thread therebetween, the magnitude of said tensioning being proportional to the magnitude of said electrical current; and means to vary said electrical current to said electromagnetic assembly to thereby vary said needle thread tension.
 8. The thread tensioning system of claim 7 wherein said means for tensioning the needle thread includes: a. said sleeve having a central bore; b. a plug mounted in the core of said sleeve near the end of said sleeve proximate said first bushing; c. an adjusting bolt threadably mounted through the other end of said casing proximate said first bushing, said screw bearing against said plug for increasing or decreasing the gap between said casing and said first bushing to vary the magnetic coupling therebetween.
 9. The thread tensioning system of claim 7 wherein said first and second bushing end plates are Teflon coated for minimizing friction between said plates and said case.
 10. The thread tensioning system of claim 7 wherein said means for supplying electrical current to said electromagnet coil of said means for tensioning the needle thread, includes: a. a variable output d.c. power supply; and b. an ammeter connected in series with said power supply and said electromagnet coil, said ammeter being used to indicate the tension being applied by said thread tensioning means upon said needle thread at any given time.
 11. The thread tensioning system of claim 10 wherein said means for supplying electrical current is connected to a plurality of electromagnet coils of thread tensioning systems similar to said needle thread tensioning means to equalize the needle thread tensions amongst such systems.
 12. An apparatus for tensioning a thread comprising: an electromagnetic assembly having a. a central ferromagnetic sleeve; b. an electromagnetic coil wound around a portion of and rigidly secured to said sleeve; c. a ferromagnetic first bushing end plate radially oriented and rigidly secured to one end of said sleeve proximate an end of said coil; d. a second bushing end plate radially oriented and rigidly secured to an interior portion of said sleeve proximate the other end of said coil; a ferromagnetic casing enclosing said electromagnetic assembly, said casing being slideably mounted upon said sleeve and said first and second bushing plates, with an end portion of said sleeve projecting therefrom; a thread guide means assembly mounted upon said projecting portion of said sleeve including: a. an outer plate rigidly attached to an end of said casing, and slideably mounted on said projecting portion of said sleeve; b. an inner plate adjacent said outer plate; c. a spacer adjacent said inner plate, at least one of said inner plate and said spacer being ferromagnetic; means for supplying electrical current to said electromagnetic assembly to create a magnetic flux passing through a magnetic circuit including said electromagnetic assembly, said sleeve, said casing and at least one of said spacer and said inner plate, causing said casing to urge said outer plate against said inner plate to tension said thread therebetween, the magnitude of said tensioning being proportional to the magnitude of said electrical current; and means to vary said electrical current to said electromagnetic assembly to thereby vary said thread tension.
 13. The thread tensioning apparatus of claim 12 wherein said thread tensioning apparatus further includes: a. said sleeve having a central core; b. a plug mounted in the core of said sleeve near the end of said sleeve proximate said first bushing; c. an adjusting bolt threadably mounted through the other end of said casing proximate said first bushing, said screw bearing against said plug for increasing or decreasing the gap between said casing and said first bushing to vary the magnetic coupling therebetween.
 14. The thread tensioning apparatus of claim 12 wherein said first and second bushing end plates are Teflon coated for minimizing friction between said plates and said case.
 15. The thread tensioning apparatus of claim 12 wherein said means for supplying electrical current to said electromagnet coil further includes: a. a variable output d.c. power supply; and b. an amMeter connected in series with said power supply and said electromagnet coil, said ammeter being used to indicate the tension being applied by said thread tensioning means upon said thread at any given time.
 16. The thread tensioning apparatus of claim 15 wherein said means for supplying electrical current is connected to a plurality of electromagnet coils of thread tensioning apparatus similar to said thread tensioning apparatus to equalize the thread tensions amongst such apparatus. 